Is the Matrix-Flow Problem Compounded by Crossflow?

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Once fractures and fracture-like features are eliminated as possibilities, the problem is deduced to be radial in nature (i.e., radial flow exists in the matrix rock around the wellbore). Next, the possibility of crossflow between reservoir strata must be addressed. If fluids can crossflow between adjacent water and hydrocarbon strata (and flow is radial), a gel treatment should not be attempted.22 Even if gelant is only injected into a single zone, it will crossflow into and damage the oil producing zones away from the wellbore. Thus, no matter how much gelant is injected, the treatment will be ineffective in promoting conformance.22 In contrast, if fluids cannot crossflow between zones and sealing Portland cement exists that prevents vertical flow immediately behind the casing, a gel treatment can be effective if gelant injection is placed only in the offending water zones.17

Several methods are used to assess whether crossflow exists between strata, including

  1. pressure tests between zones,
  2. various logs for determining fluid saturations, permeability, porosity, and lithology,
  3. injection/production profiles,
  4. simulation, and
  5. seismic methods.
The most straightforward method tests pressure differences between zones. Commonly, a packer is placed between two zones and one of the zones is allowed to pressure up. If a significant pressure can be maintained across the packer, effective barriers to crossflow exist between the zones. If a pressure difference cannot be maintained, crossflow between the zones may occur. If the operator does not know whether crossflow occurs, he should assume that crossflow exists. Ref. 23 describes an interesting case where pressure testing was used to assess the presence of crossflow.